1. Field of the Invention
The present invention relates to a semiconductor amplifier for use in a microwave band or a millimeter wave band, and more particularly to a semiconductor amplifier including a distortion compensating circuit.
2. Description of the Related Art
Systems using a radio frequency such as a microwave, a millimeter wave or the like employ a semiconductor amplifier having a non-linear amplifying element such as a Field Effect Transistor (hereinafter referred to as FET) for performing power amplification for a radio frequency. A distortion characteristic of a semiconductor amplifier has a significant effect upon the performance of the entire system. In a prior art semiconductor amplifier, the distortion characteristic is improved by connecting a distortion compensating circuit such as a linearizer to a non-linear amplifying element to remove the distortion characteristic caused by non-linearity of the element.
FIG. 1 shows a configuration of such a prior art semiconductor amplifier. Description is made using an FET as an example of a non-linear amplifying element. The prior art semiconductor amplifier comprises linearizer 21, passive elements 22, 23, output matching circuit 4, and FET 1.
Signals received at input terminal 101 are applied to a gate electrode of FET 1 through passive element 23, linearizer 21, and passive element 22. FET 1 has its source electrode grounded and its drain electrode connected to output terminal 102 through output matching circuit 4.
Linearizer 21 has a characteristic that the gain thereof is increased and an amount of a phase shift is increased in a negative direction as applied power is increased. Linearizer 21 may be a combination of passive elements, such as a coupler and an active element, such as a diode, or the like.
Passive elements 22, 23 are isolators for example, and are intended for removing reflected components caused by mismatch between linearizer 21 and FET 1. Output matching circuit 4 serves to achieve matching between FET 1 and a load connected to output terminal 102.
In the prior art semiconductor amplifier, signals received at input terminal 101 are applied to the gate electrode of FET 1 through passive element 23, linearizer 21, and passive element 22, and are then amplified at FET 1 to be finally fed to output terminal 102 through output matching circuit 4.
In the prior art semiconductor amplifier, with an increase in applied power, FET 1 serving as a non-linear amplifying element exhibits reduced gain, a deteriorated phase characteristic (that is, a phase shift is increased in a positive direction), and increased distortion. However, the prior art semiconductor amplifier is provided with linearizer 21 having a passing phase characteristic and a passing gain characteristic opposite to those of FET 1, thereby canceling the characteristics of FET 1 with the characteristics of linearizer 21 and achieving a low distortion characteristic for the entire semiconductor amplifier.
When linearizer 21 is configured by an active element such as a diode in the prior art semiconductor amplifier, power is required for driving the active element.
Also, when linearizer 21 is connected to the semiconductor amplifier, the entire size of the semiconductor amplifier is increased. Additionally, mismatch is likely to occur between linearizer 21 and FET 1 serving as a non-linear amplifying element, which requires passive elements 22, 23 such as an isolator for removing reflected components due to the mismatch, resulting in an increased size of the semiconductor amplifier.